Chemical mechanical polishing (CMP) is being increasingly used in the manufacturing of integrated circuits for dielectric planarization and metal polishing processing steps. The single most difficult problem with the CMP process in the commercial manufacturing environment are maintaining a stable material removal rate and maintaining uniformity of material removal on each wafer when processing hundreds of wafers with a single polishing pad.
The CMP process has a tendency to polish faster (i.e., remove material faster) towards the edges of a wafer. Polishing pad conditioning with abrasive surfaces and use of a fixed wafer backside pressure (sometimes called backpressure) have been suggested and used in the past for long term stability of the CMP process.
Referring to FIG. 1, in a conventional CMP system 100, a wafer 102 whose top surface 104 is to be polished is held in an inverted position by a carrier assembly 106 and polishing arm 108. The wafer 102 is held in position against a rotating polishing pad 110 which removes material from the top surface 104 of the wafer 102 from mechanical abrasion from the polishing pad 110 and particles in the slurry and from chemical action from the slurry on the polishing pad 110. Rotation of the polishing pad during the polishing process is caused by a motor 112, while rotation of the wafer is caused by another motor 114. In addition there is a periodic translation motion by the polishing arm 108 so as to use different portions of the polishing pad over time.
The carrier assembly 106 is a pneumatic carrier that is connected to a vacuum pump and air pressure pump assembly 120 via tubing (not shown) in the polishing arm 108. The pneumatic carrier includes a perforated steel plate 122 and a capture ring 124, which together hold a perforated carrier pad 126 and the wafer 102. When a wafer is first moved by an automated wafer transportation system 130 from a wafer transport tray 132 to the carrier assembly 106, a vacuum pump is coupled to the carrier assembly 106 so as to hold the wafer in place while the polishing arm moves the wafer into position adjacent the polishing pad 110. After the wafer is in position and polishing begins, backside pressure can be applied to the wafer by (A) downward movement of the polishing arm 108 and (B) application of positive air pressure through the perforated metal carrier plate 122 and carrier pad 126.
Slurry is dispensed onto the polishing pad 110 through a slurry inlet 128. A pad conditioning system 140 conditions the polishing pad with abrasive surfaces. Conditioning the pad makes it rough, which allows effective application of downward force and also causes the pad to act as a conduit for slurry flow to the wafer surface.
It is a goal of the present to improve, in a chemical mechanical polishing system for processing semiconductor wafers, the stability of the material removal rate and to improve the uniformity of material removal on each wafer when processing hundreds of wafers with a single polishing pad and a single carrier pad.